cros: Remove unused low memory margin code from sandbox and zygote

content/public/browser/zygote_host_linux.h

Index: content/public/browser/zygote_host_linux.h
diff --git a/content/public/browser/zygote_host_linux.h b/content/public/browser/zygote_host_linux.h
index a9798de10dc87e59d0cac0a6fc21225953420b74..77cc00b2fb1a5253915508903dcdac0d5ec3631e 100644
--- a/content/public/browser/zygote_host_linux.h
+++ b/content/public/browser/zygote_host_linux.h
@@ -38,12 +38,6 @@ class ZygoteHost {
   // likely to be killed by the OOM killer.
   virtual void AdjustRendererOOMScore(base::ProcessHandle process_handle,
                                       int score) = 0;
-
-  // Adjust the point at which the low memory notifier in the kernel tells
-  // us that we're low on memory.  When there is less than |margin_mb| left,
-  // then the notifier will notify us.  Set |margin_mb| to -1 to turn off
-  // low memory notification altogether.
-  virtual void AdjustLowMemoryMargin(int64 margin_mb) = 0;
 };
 
 }  // namespace content